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roof pitch
- Thread starter Sifu
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ICE
Oh Well
Requires engineering here.Sifu said:
north star
Sawhorse
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Yes, ...refer to Section R802.2 in the `06 IRC......Also, see SectionR905 for the types of roof coverings when the roof pitch is not 3/12.
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Sifu
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Those are the same sections I looked at, just wanted to verify. Guy has a 1 1/2 with metal roofing. Told him that 1) code requirements olnly applied to framing 3/12 or greater and 2) his metal roofing, depending on the type and specs probably wouldn't allow such a low slope. He is checking the roof material and I checked the framing requirement. Job done.
Thanks
Thanks
ICE
Oh Well
I agree with brudgers up to a point. A shed roof could rely on a table but a low sloped gable roof has a beam for a ridge and a particular connection of the rafter and tie.
ICE
Oh Well
We have a handout for carports that has a span up to 25'. Most shed roofs get by without engineering.
Sifu
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I'm getting some mixed opinions. This is a gable roof with the ridge offset which makes the one side such a low slope. Code says 3/12 but is doesn't explicitly say a design is required otherwise, it seems a little vague. I thought of treating it like a floor but didn't know if that was a reasonable option and with the reduced span allowance of a floor joist he won't have a center bearing location.......maybe if I use the 30lb load for sleeping areas I can get to a bearing location. I would need to check that. This is a residential addition, poor guys had no idea. I would like to find a reasonable compromise. As far as the roof panel goes, some metal panels can go down on low slopes but some are restricted to 3/12. I think his is just a corrugated lap panel with no lap sealant but I'll let the mfr determine the slope they require. Is there a hazard I'm not aware of in looking at this as a sleeping room floor joist? There is no ridge beam, just a 2x10. I am requiring adequate rafter ties but the code says any beam supporting less than 3/12 be designed. If I use rafter ties instead of a ridge beam does that code not apply?
If only I were allowed to look at plans........
If only I were allowed to look at plans........
The tie tension at low slopes becomes very high, look at the heeljoint table at the end of the rafter span tables. A ridgebeam rather than a ridgeboard is needed. With a ridgebeam no rafter tie is needed, this is how a cathedral roof with no ties is constructed.
This will give you some idea of the forces that would need to be restrained in a symmetrical gable, try a 12/12 pitch, 1.5/ 12, and then for kicks .5/12 and .05/12
http://www.timbertoolbox.com/Calcs/RafterThrust.htm
The nails in a typical tie are going to be good for 75-100 lbs each, he'll probably run out of wood before he gets the tie connected well enough and at that point may be getting into the tensile limits of the tie lumber itself. It needs a ridgebeam, with correct supports. Any ridgeBEAM needs design from somewhere.
This will give you some idea of the forces that would need to be restrained in a symmetrical gable, try a 12/12 pitch, 1.5/ 12, and then for kicks .5/12 and .05/12
http://www.timbertoolbox.com/Calcs/RafterThrust.htm
The nails in a typical tie are going to be good for 75-100 lbs each, he'll probably run out of wood before he gets the tie connected well enough and at that point may be getting into the tensile limits of the tie lumber itself. It needs a ridgebeam, with correct supports. Any ridgeBEAM needs design from somewhere.
ICE
Oh Well
Sifu,
Once you go below 3" pitch the ridge is a beam. As DRP pointed out, the beam may not need a tie. On the other hand, if a tie is available, a smaller beam will work.
DRP,
The calculator that you provided is worth posting at the Useful Links Forum.
Once you go below 3" pitch the ridge is a beam. As DRP pointed out, the beam may not need a tie. On the other hand, if a tie is available, a smaller beam will work.
DRP,
The calculator that you provided is worth posting at the Useful Links Forum.
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dhengr
Bronze Member
Sifu:
The mix-up is that you didn’t define your problem very well in your OP. DRP’s rafter thrust calculator is for a symmetrical gable roof system. It will show you how trust is related to roof slope all else being equal. Your’s is not symmetrical, with two different rafter slopes and a ridge beam which is not in the center of the bldg. This will lead to different vertical reactions and thrusts on the rafters on each side of the ridge and thus the horiz. thrust on the ridge beam is different on each side, and unbalanced, on the ridge beam. This situation is best handled with a load carrying and spanning ridge beam, not rafters and a ridge board; and treating the rafters as beams too, as if they were fl. jsts. and only impart vertical reactions to the ridge beam and ext. walls. This roof configuration usually requires some engineering attention. One of the first times I meet DRP on this forum was on a thread which was essentially this very same low slope roof problem. And, he is right on the money when he says low sloped rafters lead to very high thrusts, which can not be easily accommodated at the ext. walls, or with clg. jst. ties. The main reason for the IRC limiting the rafter slope to 3/12 is this high thrust problem, but your’s is unbalanced too.
The mix-up is that you didn’t define your problem very well in your OP. DRP’s rafter thrust calculator is for a symmetrical gable roof system. It will show you how trust is related to roof slope all else being equal. Your’s is not symmetrical, with two different rafter slopes and a ridge beam which is not in the center of the bldg. This will lead to different vertical reactions and thrusts on the rafters on each side of the ridge and thus the horiz. thrust on the ridge beam is different on each side, and unbalanced, on the ridge beam. This situation is best handled with a load carrying and spanning ridge beam, not rafters and a ridge board; and treating the rafters as beams too, as if they were fl. jsts. and only impart vertical reactions to the ridge beam and ext. walls. This roof configuration usually requires some engineering attention. One of the first times I meet DRP on this forum was on a thread which was essentially this very same low slope roof problem. And, he is right on the money when he says low sloped rafters lead to very high thrusts, which can not be easily accommodated at the ext. walls, or with clg. jst. ties. The main reason for the IRC limiting the rafter slope to 3/12 is this high thrust problem, but your’s is unbalanced too.
ICE, it is real hard for me to disagree with the guy from Tarsus, I apologize.
The comment about a tie reducing the beam size is one that gave me heartburn on a job I visited recently. The crew deviated and raised a set of ties well above halfway and installed an lvl that was too small to be called a ridgebeam. Their logic was that they were helping one another. My logic was that the beam will deflect and the load will then fall on the ties. Instead of one helping the other I believe the ties will fail, and then the undersized beam will fail. I could be wrong, that's engineer country and I'd love to hear from one. I'd much rather be able to point to a brutally basic load path, make one capable of fully taking the load and call the other redundancy or decoration.
The comment about a tie reducing the beam size is one that gave me heartburn on a job I visited recently. The crew deviated and raised a set of ties well above halfway and installed an lvl that was too small to be called a ridgebeam. Their logic was that they were helping one another. My logic was that the beam will deflect and the load will then fall on the ties. Instead of one helping the other I believe the ties will fail, and then the undersized beam will fail. I could be wrong, that's engineer country and I'd love to hear from one. I'd much rather be able to point to a brutally basic load path, make one capable of fully taking the load and call the other redundancy or decoration.
dhengr
Bronze Member
I think DRP has it exactly right, without putting exact numbers on the problem. When you raise the ties you increase the tie tension forces (thrust reactions) for equilibrium of the roof system, thus making the tie to rafter connection more difficult to make adequately and more likely to be a failure mechanism. This higher tie also induces new axial and bending loads into the rafter, which causes it to need to be increased in sized. The cleanest and most direct load path solution is to make the ridge beam a spanning member, and to treat what had been low slope rafters as simple beams spanning from the ext. wall to the ridge beam. They no longer cause a thrust, so the large thrust problem goes away. By the way the rafters are cut or fitted, they only apply vert. reactions to the walls and the ridge beam.
I think I understand ICE’s or DRP’s builder friend’s thought process, and it goes something like this: if you can only make the rafters act a little bit like normal rafters, they will either impart less reaction on the ridge beam, or actually act like a normal rafter and actually balance and support the ridge beam (now a ridge board); in either case you should be able to reduce the size of the ridge beam, maybe only to a ridge board. However, the rafter/tie connection can’t be desinged/built half way, it slips/yields a little, allowing some rafter spreading and putting more vert load on the lighter ridge beam, which deflects some more; which causes more spreading of the rafters, or higher tie tension and finally, I would bet on the tension tie/rafter connection failing. These members and member sizes and connections can be made to work, within reason, but they are just not normally practical. This is akin to a very flat sloped truss, which really act like a beam, so design it as a beam or parallel chord truss; or akin to a very flat arch which has such high thrusts that it becomes an impractical load carrying system.
Structures don’t act like you wish they would act, they act the way your design and detailing force them to act, and they are smart enough to know how they must act. You can’t fool them, but they sure as hell can fool you, if you’re not careful.
I think I understand ICE’s or DRP’s builder friend’s thought process, and it goes something like this: if you can only make the rafters act a little bit like normal rafters, they will either impart less reaction on the ridge beam, or actually act like a normal rafter and actually balance and support the ridge beam (now a ridge board); in either case you should be able to reduce the size of the ridge beam, maybe only to a ridge board. However, the rafter/tie connection can’t be desinged/built half way, it slips/yields a little, allowing some rafter spreading and putting more vert load on the lighter ridge beam, which deflects some more; which causes more spreading of the rafters, or higher tie tension and finally, I would bet on the tension tie/rafter connection failing. These members and member sizes and connections can be made to work, within reason, but they are just not normally practical. This is akin to a very flat sloped truss, which really act like a beam, so design it as a beam or parallel chord truss; or akin to a very flat arch which has such high thrusts that it becomes an impractical load carrying system.
Structures don’t act like you wish they would act, they act the way your design and detailing force them to act, and they are smart enough to know how they must act. You can’t fool them, but they sure as hell can fool you, if you’re not careful.
Sifu
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OK, so back to the original. Sorry I didn't provide more info in the OP. I try to keep it simple in order to get a simple answer......of course that usually isn't that simple. I figured he had two possible options, 1) a ridge beam 2) a floor system. I see now a floor system won't work prescriptively even with ties. I'll inform him he will need a ridge beam designed but will that do it?
I quote dhengr- "The cleanest and most direct load path solution is to make the ridge beam a spanning member, and to treat what had been low slope rafters as simple beams spanning from the ext. wall to the ridge beam. They no longer cause a thrust, so the large thrust problem goes away. By the way the rafters are cut or fitted, they only apply vert. reactions to the walls and the ridge beam."
Would the rafters now need to be designed as beams or just sized as floor joists? Currently they are over-spanned on the low slope side but there is a wall I can bear them on which would reduce the span to an allowable length for that of a sleeping room.
Given the unbalanced nature of the roof would it still just be simple vertical load on the beam and if so could the beam be retrofitted under the existing ridge board and rafters?
I quote dhengr- "The cleanest and most direct load path solution is to make the ridge beam a spanning member, and to treat what had been low slope rafters as simple beams spanning from the ext. wall to the ridge beam. They no longer cause a thrust, so the large thrust problem goes away. By the way the rafters are cut or fitted, they only apply vert. reactions to the walls and the ridge beam."
Would the rafters now need to be designed as beams or just sized as floor joists? Currently they are over-spanned on the low slope side but there is a wall I can bear them on which would reduce the span to an allowable length for that of a sleeping room.
Given the unbalanced nature of the roof would it still just be simple vertical load on the beam and if so could the beam be retrofitted under the existing ridge board and rafters?
ICE
Oh Well
I get the feeling that you think I am that guy before his conversion. That I can fix.
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ICE, I was raising my hand as guilty too. I think we have both knocked the rough edges off and found a better honing angle 
Sifu,
This is unrelated to the thread but related to the conversation. This calc is crude and my graphic could use some work for clarity, but shows the tension force in a raised tie and lets you play with that to gain more understanding of the forces at work. I like writing these to play "what if" and further my understanding. As dhengr noted that tie point is also inducing further bending in the rafter that must be accounted for, check the footnotes under the rafter tables.
http://www.timbertoolbox.com/Calcs/raisedtiethrust.htm
dhengr, I just spot checked the awc spancalc. I believe the rafter tables do treat the rafter as a simple beam. I think the differences in the span tables is due to load duration from floor to snow in the allowable design values. If I'm correct in that, I believe the rafter tables are appropriate. We still have a ridgebeam problem that I hope you'll hold class on.
Sifu,
This is unrelated to the thread but related to the conversation. This calc is crude and my graphic could use some work for clarity, but shows the tension force in a raised tie and lets you play with that to gain more understanding of the forces at work. I like writing these to play "what if" and further my understanding. As dhengr noted that tie point is also inducing further bending in the rafter that must be accounted for, check the footnotes under the rafter tables.
http://www.timbertoolbox.com/Calcs/raisedtiethrust.htm
dhengr, I just spot checked the awc spancalc. I believe the rafter tables do treat the rafter as a simple beam. I think the differences in the span tables is due to load duration from floor to snow in the allowable design values. If I'm correct in that, I believe the rafter tables are appropriate. We still have a ridgebeam problem that I hope you'll hold class on.